There is known a system (cf. A. A. Kuznetsov "Vibration Tests of Automatic Elements and Devices", Moscow, "Energy" publishers, 1976, p. 107, in Russian) for testing articles with wide-band random vibrations by scanning a random narrow-band signal in a predetermined frequency range, said method enabling tests of articles by affecting them with a scanning harmonic signal also in a predetermined frequency range. The system comprises such series-connected components as a white-noise generator, a narrow-band filter, a balanced modulator having its second input connected to a crystal oscillator, an adjustable amplifier having its second input connected to such series-connected components as an amplifier and a detector of the automatic gain control circuit, a mixer having its second input connected to a variable-frequency oscillator, and an output amplifier connected to the input of a vibrator and to the input of a voltmeter.
The narrow-band filter may be transferred to the harmonic signal generating mode for testing articles with harmonic vibration.
The aforesaid system has been open to the objection that it is generally impossible, firstly, to produce wide-band random vibrations throughout the preset frequency range and, secondly, to perform tests with mixed harmonic and random vibrations.
There is also known a system (cf. relevant publication by Bruel & Kjaer, Denmark) for testing articles with mixed harmonic and random vibrations comprising a multichannel random signal shaper wherein each channel includes such series-connected components as a noise generator, a bandpass filter and an adjustable amplifier, the output of each channel being connected to one of the inputs of an adder, and a vibrator with a vibration pick-up for converting mechanical oscillations into an electrical signal, which is connected to the multichannel random signal shaper through such series-connected components as a second adder and a power amplifier. The output of the vibration pick-up is connected through a matching amplifier to the input of a tuneable filter having two outputs, one of which (namely, the suppression output) is connected via a multichannel random signal spectrum analyzer to the adjustable amplifier of each channel of the multichannel shaper, while the other output is connected to the control input of a harmonic signal generator whose harmonic tuning frequency corresponds to the tuning frequency of said tuneable filter, the output of said generator being connected to the input of the second adder.
The system under review permits testing articles simultaneously with harmonic and random vibrations. However, as a harmonic signal is automatically passed through a predetermined frequency range, there occurs synchronous tuning of the tuneable filter whose suppression output is connected to the input of the random signal spectrum analyzer. Consequently, the multichannel random signal spectrum analyzer will measure a decreased value in the nth channel instead of true variance of a random signal, D.sub.n, said signal being formed by the multichannel random signal shaper in the nth channel. The above condition is caused by the need to suppress a harmonic signal by .DELTA.D.sub.n, which indicates a shaping error in the random variation spectrum in the nth band of the predetermined frequency range. Said shaping error will result in increased vibrations.
Another disadvantage of the aforesaid system is a large measuring error and, in effect, an appreciable error in setting a harmonic signal level due to the fact that the random signal spectrum overlap, particularly when the random signal is commensurable with the harmonic signal level or exceeds it. Moreover, the random signal level is channels of the random signal shaper is controlled with due account taken of the results obtained in measuring spectral density of the random signal, which gives rise to a statistical analysis error ##EQU1## where T is the analysis time determined by parameters of the random signal spectrum analyzer and .DELTA.f is the band wherein the frequencies are analyzed. Decreasing .epsilon. increases an analysis time, a disadvantage substantially increasing the time required to tune the multichannel random signal shaper.